Redact summaries in fsstore via shared checkpoint.RedactSummary

Final review caveat: the git store redacts checkpoint summaries before storing them, but fsstore stored opts.Summary / SessionSummary directly. Export checkpoint.RedactSummary (was the private redactSummary) and use it in fsstore's session write and summary write so the reference backend models the same redaction the git store performs. Broaden the package doc to note the remaining intentional gaps (cross-session token aggregation, derived CLI/skill-events version stamps).